High levels of whiteness and brightness are important parameters for the end-user of paper products. The most important raw materials of the papermaking industry are cellulose, pulp and lignin which naturally absorb blue light and therefore are yellowish in color and impart a dull appearance to the paper.
The distinction between whiteness and brightness is well-known to those skilled in the art and is discussed, for example, in WO 0 218 705 A1.
Optical brighteners are used in the papermaking industry to compensate for the absorption of blue light by absorbing UV-light with a maximum wavelength of 350-360 nm and converting it into visible blue light with a maximum wavelength of 440 nm.
It is well established that, in addition to optical brighteners, certain shading dyes or pigments can be added to the paper in order to achieve a higher level of whiteness and to control the shade of the white paper.
WO 0 218 705 A1 however teaches that the use of shading dyes or pigments, while having a positive effect on whiteness, has a negative effect on brightness. The solution to this problem is to add additional optical brightener, the advantage claimed in WO 0 218 705 A1 being characterized by the use of a mixture comprising at least one direct dye (exemplified by CI Direct Violet 35) or pigment and at least one optical brightener.
Surprisingly, we have now discovered a sizing composition comprising an optical brightener and a shading dye which enables the papermaker to reach a high level of whiteness without significant loss in brightness.
Therefore, the goal of the present invention is to provide aqueous sizing compositions containing derivatives of diaminostilbene optical brightener, shading dyes, binders and optionally divalent metal salts affording enhanced high whiteness levels while avoiding the loss of brightness characterized by the use of shading dyes or pigments when applied to the paper at the size press.
The present invention further provides a process for optical brightening and tinting paper substrates characterized in that an aqueous sizing composition containing at least one optical brightener, at least one shading dye, at least one binder and optionally at least one divalent metal salt is used.